1. FIELD OF THE INVENTION
The present invention relates to the preparation of methylene-bridged polyphenylpolyamines and more particularly pertains to the treatment of methylene-bridged polyphenylpolyamine mixtures prepared by the silica-alumina catalyzed condensation reaction of aniline and formaldehyde.
2. DESCRIPTION OF THE PRIOR ART
The preparation of methylene-bridged polyphenylpolyamine mixtures by the condensation reaction of aniline and formaldehyde carried out at elevated temperatures in the presence of a silica-alumina cracking catalyst is well-known in the art and has been practiced commercially for many years. See for example U.S. Pat. No. 3,362,979 to Bentley. Generally speaking, such silica-alumina catalyzed aniline-formaldehyde condensation reaction procedures result in the production of methylene-bridged polyphenylpolyamine product mixtures containing a diamine portion, i.e., diaminodiphenyl methane, in the amount of from about 30 to about 92 weight percent and the remainder being higher molecular weight and higher functionality polymethylene polyphenylpolyamines (triamines, tetramines, etc.). As known, the amount of the diamine portion is dependent upon several processing variables, especially the molar ratio of aniline to formaldehyde employed. Usually at lower aniline-formaldehyde molar ratios, such as ratios of from about 1:1 to about 2.5:1, the higher functionality, molecular weight polyamines will be formed preferentially and the yield of higher polyamines is in excess to the yield of diamine. However, as progressively larger amounts of aniline are used, the yield of diamine is progressively increased at the expense of the higher functionality, molecular weight polyamine yield.
In addition, as known, the diamine portion of a polymethylene polyphenylpolyamine mixture produced by the silica-alumina catalyzed condensation reaction is formed as a mixture of the 2,2'-, 2,4'- and 4,4'-diamine positional isomers. The actual distribution between the positional isomer forms can be somewhat regulated by controlling processing variables such as temperature, method of addition of reactants and the like which are very well known. However, the diamine portion of such reaction product mixtures normally contains at least some portion of each positional isomer.
Diaminodiphenyl methane mixtures having high 4'-positional isomer content are particularly valuable materials as precursors for the preparation of corresponding methylene diphenylisocyanates which are useful in the preparation of certain types of coatings, elastomers and the like. Usually, such diaminodiphenyl methane materials are obtained from methylene-bridged polyphenylpolyamine reaction product mixtures prepared by the condensation reaction of aniline and formaldehyde carried out in the presence of a strong mineral acid catalyst, e.g., hydrochloric acid. Mineral acid catalyzed condensation reactions normally result in the production of aromatic polyamine reaction products having diamine portions which contain high amounts of the 4,4'-positional isomer. However, such mineral acid catalyzed condensation reaction procedures suffer from the disadvantage of requiring a neutralization step with a basic material, such as caustic, which accordingly brings on the difficulties of by-product removal and disposal. Moreover, mineral acids are generally highly corrosive and condensation reactions using them as catalysts require the utilization of expensive corrosive-resistant equipment such as glass-lined reactors and the like.
The above-mentioned silica-alumina catalyzed condensation reaction procedures do not require neutralization of the reaction product or the use of expensive corrosive-resistant equipment. However, it has heretofore been considered impossible to obtain a substantially pure diaminodiphenylmethane product containing substantially high levels of the 4,4'-positional isomer, e.g. above about 85 weight percent, basis diaminodiphenylmethane, from methylene-bridged polyphenylpolyamine mixtures produced by the aforementioned silica-alumina catalyzed condensation reaction. Experiments have shown that no more than about 92 weight percent diaminodiphenylmethane can be produced by the silica-alumina catalyzed condensation reaction no matter what molar ratios of aniline to formaldehyde are employed. Moreover, separation of high 4,4'-isomer-containing diaminodiphenylmethane product from such polyamine reaction product mixtures by distillation is very difficult and sacrificial. Such distillation techniques require the employment of expensive high vacuum distillation apparatus and subjecting the polyamine product mixtures to high temperatures. As known, polymethylene polyphenylpolyamines are susceptible to heat degradation. During such a high temperature distillation, these products have been found to degrade somewhat to aniline.
We have now discovered a process for obtaining substantially pure diaminodiphenylmethane mixtures containing substantially high amounts of the 4,4'-isomer form from certain methylene-bridged polyphenylpolyamine mixtures prepared by the silica-alumina catalyzed condensation reaction of aniline and formaldehyde. The discovery is believed to be a tremendous advance in the art inasmuch as it provides a technique for obtaining the desired, valuable high 4,4'-isomer containing methylene diphenylamines (diaminodiphenylmethane) product without encountering the disadvantages observed with mineral acid catalyzed condensation procedures. Moreover, the discovery provides enhanced flexibility for the use of polymethylene polyphenylpolyamine mixtures prepared by silica-alumina catalyzed condensation reaction procedures. Not only can substantially pure diaminodiphenylmethane products containing high levels of the 4,4'-isomer be obtained but also polymethylene polyphenylpolyamine mixtures of low diamine content and having narrow molecular weight distributions can be obtained which are useful as precursors for the preparation of corresponding polymethylene polyphenylisocyanates that can be employed in the production of specialty elastomers, coatings and the like.